Radio receiver



c. B. H. FELDMAN 2,233,384

RADIO RECEIVER Filed Dec. 28. 1939 Feb. 2 5, 1941.

ATTORNEY l Patented Feb. 25, 1941 l UNITED STATES PATENT OFFICE RADIORECEIVER Carl B. H. Feldman, Rumson, N. J., assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication December 28, 1939, Serial No. 311,271

7 Claims. (C1. Z50-20) This invention relates to signaling systems thoseof the exact frequency of the received carsuch as radio telephone andtelegraph systems rier would be eliminated. However, it has been andparticularly to radio systems in which a sinfound that in the absence ofa received carrier, gle frequency oscillation such as a carrier is thereoccurs false operation which is not eX- utilized. plainable upon thebasis of noise currents of the 5 An object of the invention is toeliminate or recarrier frequency. duce the effect of noise on theoperation of such Applicant has demonstrated that this eifect issystems. due to the band of noise currents passed by the In radiotelephone systems, certain types of earlier selective circuits of thesystem. Thus, the control equipment are operated by the received iirstmodulation process will produce the same 10 carrier wave. One example isthe so-called frequency translation of the noise currents in Codan(carrier-operated-device anti-noise) in the intermediate frequency bandas that prowhich the received carrier wave is utilized for duced on thecarrier. The side-band output of rendering operative a normally blockedradio rethis modulator will therefore contain an oscillal? ceiver. Insuch systems, it is necessary to protion corresponding to the carriersurrounded by 15 vide high selectivity of the carrier wave to prenoisecurrents corresponding in relative frequenvent false operation by noisecurrents. I-Iowcies and phase to those in the intermediate freever, vthereceived wave may vary in frequency quency band fed to the modulator.The filter because of transmitter variations, fading or the used forselecting the desired side-band output .20 like, consequently theselective circuits must cannot be too highly selective as the frequencyaccommodate a band wide enough to take care of the desired modulationproduct representing of such variations. As a result noise currents thecarrier will Vary in absolute value in the will also be admitted so thatnoise cannot be same Way as the frequency of the received Wave.completely discriminated against by frequency If the filter is somewhatnarrower than the first '25 selectivity. intermediate frequency filter,some of the noise 25 The effect of frequency variations can berecomponents may be removed. However, those duced to permit the use ofhigher selectivity by within the pass-band will remain. the use ofautomatic tuning systems or the equiv- The corresponding noiselcomponents in ,the alent. One type of system for accomplishing twobranches supplied to the second modulator this purpose is that disclosedand claimed in will, of course, be separated by a frequency cor- 30United States Patent 2,093,871 to Levin vof Sepresponding to thefrequency of the oscillation tember y21, 1937. supplied to the firstmodulator. Intermodula- This type or system makes use of a multiple tionof each pair of corresponding noise curmcdulation process. Theintermediate frerents will therefore produce a modulation prodquencyoutput of the first detector of the superuct of that frequency. Theseresultant currents 35 heterodyne radio receiver is fed to a iirstcmodnwill tend to add together vectorially in phase soi later in whichit is combined with the output-l of that in the absence 0f some Specialrrfmaution,k a fixed frequency oscillator. One modulation the noisecurrents will produce a pseudo-carrier product (sum or difference) isselected in the of the same frequency as that produced by the .1 0Output 0f this modulator and Supplied to. a seeaction of the two carrierwaves. Because its fre- 40 ond modulator. The original intermediatefrequency is the same as that of the carrier, this quency is alsosupplied to this second modulator. pseudo-carrier cannot be eliminatedby the nar- One of the resultant products of this second row filterconnected to the loutput of the second modulator will be an oscillationof frequency modulator. The result is that even in the abequal to thefrequency of the oscillations from sence of a received carrier, thenoise currents 45 the iixed frequency oscillator used in the first WillDI'OUC@ a Current 0f the Same frequency and modulation process'. Thisoscillation may be seconsequently the same elfect as the received lectedin a narrow iilter since it is of fixed frecarrier. y quency dependingonly upon the stability of the In accordance with a feature of thepresent fixed frequency oscillator used with the first invention, theeifect of noise just discussed is 50 modulator. Where `only a singlefrequency oseliminated by the introduction of a differentialcillationrepresenting the carrier is desired for phase shift or timedelay between the currents some control operation, the filter band maybe fed to the second modulator over the two paths. made extremelynarrow. As a result it would By a proper control of the phase of thenoise I n5 appear that the effect of all noise currents., exceptmlrrentsy from .these two paths, the component 55 modulation productscan be made to add in such phases that their effect is not cumulative.

More specifically, the object of the present invention is accomplishedby providing such a phase shift for each noise component in one band ofoscillations fed to the second modulator that the component modulationproducts representing the combination of corresponding noise componentsof the two bands will not add in like phase; To accomplish this theremust be produced a phase shift in one of the bands which varies acrossthe band as distinguished from a phase shift which is the same for allfrequencies in the band. Assuming that the noise currents in thefrequency band are of equal amplitude, the vector addition of themodulation components will result in zero for a phase shift in onebranch differing from that in the other by 21|', 41|, etc. radians overthe band. This corresponds to a diiference in transmission delay of thetwo ,branches of l etc., seconds, where 'F is a band width in cycles persecond.

T'he features of this invention as just described result in theelimination of the pseudo-carrier. However, this current does not takethe place of the original noise but appears in addition thereto.yFurther, the second modulation process may also produce noise currentsof frequencies close to that of the carrier by the intermodulation ofnoise currents in the two "branches differing by nearly the carrierfrequency. Such pairs of cur-4 rents are, of course, of random phase andcannot 'be made more random by phase control.

These land other features and aspects of the invention may be morereadily understood by reference to the following detailed description inconnection with the drawing in which:

Fig. 1 is a block schematic circuit showing the invention; Fig. 2 is asingle line, block schematic circuit of la radio receiver employing theinvention in a -codan circuit thereof; and

Fig. 3 is a` graph illustrating the operation of the invention.- lvReferring to Fig. 1, there is shown a source of oscillations 5 which-mayin the case of a radio receiver, be either the received oscillationsor the intermediate frequency. The frequency of the "output of thesource of the oscillator 5 is represented as f1 iA'where f1 is thenominal frequency land A-is the frequency variation.

The oscillations from the source-5 are fed to two paths kIi and 'I. Inthe pathfl the oscillations vare transmitted through a delay network 8to one input of a modulator 9. oscillations of frequency f2 from aconstant frequency oscillator I are also supplied to the modulator 9, Afilter f-II'is connected to''the output of the modulator 9.jilih'islter'is designed to select one of the side- Slban'ds ofthemodulationprocess to the exclusion of therf`-frequenfcies-if Therewilltherefore apreceived or intermediate frequency represented by the sourceappear in both of the paths B and 'I, the output of the second modulatorI2 contains a component of fixed frequency f2 without such variations.This is due to the double modulation process in which the variationscancel out.

The filter II must be capable of taking care ofthe frequency variations(A) in the oscillations from the source 5 and consequently must have aband width 2A. As a result there will appear in the output of the filterII, not only the oscillation representing the incoming signal (f1-LA)but also noise currents lying within the 2A frequency band. These noisecurrents represent the original noise currents accompanying the receivedsignal. The effect of the modulation process in the modulator 9 is toproduce the same frequency translation of the noise currents as isproduced on the signal Wave. Each component of the noise currents in theoutput of filter II therefore corresponds to a similar lnoise current inthe circuit Ybranch 6 and the corresponding components in the twobranches are separated in frequency by the frequency f2.

One effect of the modulation process in the modulator rI2 is to producefor each pair of corresponding noise currents of the two branches amodulation component of frequency f2. If there were no difference in thephase characteristics of the two branches Ii and 1, these resultantcomponents would add vectorially so as to produce a cumulative outputof` frequency f2 of substantial magnitude even when the individual noisecurrents which go to produce it are comparatively small. With such asystem, therefore, there may appear in the output of the filter I3 dueto random noise currents, a current of frequency f2, even in the absenceof current of frequency f1 from the source 5. (This pseudo-car rier andthe components which go to make it up do not take the place of the noisecurrents but appear additionally thereto.) As has been` previouslydiscussed, this pseudo-carrier may be eliminated or reduced to animmaterial value by va proper control of the relative phase constants ofthe two branches 6 and l. This is a function .of the delay network 8 Vinthe circuit of Fig 1.

While the network 8 is herein shown as an individual circuit elementconnected in the input to the modulator 9, it should be understood notonly that the particular point of its connection in the branch 1 isimmaterial, but also that it rneed not be a separate circuit element asits .function may be incorporated in other circuit .elements such as thefilter II. The only requirement is that the over-all phase constant ofthe circuit 'I differs from that of the circuit 6 in the requiredmanner. This, of course, could also be produced by the addition of anetwork.

of the'proper phase constant in the circuit 6 I instead of that branchbeing the direct connecvtion shown.

However, it will 'be found that, in general, the

desired effect can be most economically produced` by a control of thephase characteristic of the branch 1. The inherent nature of the systemrequires the use of a lter I I in this branch, and

the usual type of network'employed for such fzlters has a phasecharacteristic of the type required'f'so that it may be used toproducepartk 0r ballo'fthe required differentialA phase shift. On1th'eothr1handin the branch 6 there is no par- 5,:ticul'arnee'dforv anynetwork' which might pro- -:duce-ggdemyg As has been kstated therequired differential phase shift between the branches 6 and 1 is suchthat the components of pseudo-carrierr in the output of the modulator l2will not add vectorially to produce a cumulative effect. The par-lticu/lar requirement for the differential phase constant for thispurpose can be most' readily understood by consideration of thecondition in which the noise energy is uniformly distributed throughoutthe band, i. e., equal amplitude noise currents. It also appears thatthis is a fair assumption for the average condition of noise.-

`Under such conditions and with no difference in phase constants betweenthe two branches, the components of the pseudo-carrier would tend to addin phase and produce the maximum resultant. On the other hand if therelative phase constants of the two branches are such that there isproduced a progressive phase shift across the band (2A) such that thephase 'of the component on one extreme of the band differs from that onthe other 2nradians (or other even multiple 1r radians) it is obviousthat the resultant vector summation will be zero. The effect of variousdifferential progressive phase shifts are shown graphically in Fig. 3.In that diagram the ordinates represent the vector sum of thepseudocarrier components in the output of modulator I2 and the abscissaerepresent differential progressive phase shifts or delay across thefrequency band F=2A.

Clearly the effect of curving ther resultant vector addition of thepseudo-carrier components is achieved only by a phase shift which isprogressively greater across the frequency band of one branch than ofthe other. Uniform phase shifts of the entire band which may bedifferent in the two branches do not disturb the irl-phase vectoraddition.

Fig. 2 is a schematic circuit diagram of a radio telephone receiver ofthe superheterodyne type employing the present invention in a codancircuit. As is well understood in the art, codan circuits are used forreducing the noise in radio receivers when no signal is being received.Such circuits are particularly desirable with radio receivers employingautomatic volume control as with such receivers the gain will be soincreased during periods in which no signal is received that thereceived noise will be highly amplified so as to produce unduedisturbances in the output. In general the radio receiver is arranged tobe blocked in the absence of a received carrier and the codan circuitresponds to a received carrier to render the radio receiver operative.Obviously the codan must distinguish between noise and carrier .whichintroduces various problems particularly for use at short waves wherethe received carrier may vary considerably in frequency as well asamplitude.

In the circuit of Fig. 2, the radio signals received in the antenna 2|are selected in the tuned circuits 22 and 24 connected to the input andoutput, respectively, of a radio frequency amplifier 23. After thisselection and amplifying process the radio waves are supplied to theinput of the first detector 25 where they are combined with oscillationsfrom a beating'oscillator 26 to produce the intermediate frequencyoscillations. The intermediate frequency output of the first detector 25is selected in the intermediate frequency filter 21 amplified by theintermediate frequency amplifier 28. Additional stagesmay be `employedas is well known in the art although plied to the modulator 42.

only a single stage is shown herein for the purpose of simplicity.

The output of the intermediate frequency arnplifier is. branched off inseveral branches. Two paths, namely 33 and 31, go to the codan circuitas will be described in detail hereinafter. Another path goes to thesecond or signal detector 23m which the audio frequencies are detectedand supplied through an audio frequency ampli- Iier 3,6 to the audiofrequency circuit 3|. This circuit is normally blocked by theshort-circuiting contacts 32 but is rendered operative in response to areceived carrier wave by the operation of the codancircuit as will bedescribed.

An additional branch of the intermediate frequency circuit goes to anautomatic volume control rectifier 33, the output of which is suppliedto the radio frequency amplifier 23, first detector 25 and intermediatefrequency amplifier 28 to control the gain of those stages as is wellunderstood in the art.

The codan circuit, for the purpose of taking care of frequencyvariations in the received carrier, employs a circuit of the type shownin Fig. 1 embodying the features of the present invention. Thus theoutput of the intermediate frequency amplifier 28 is fed through theintermediate frequency filter 38 and circuit 31 to a modulator 39 inwhich the intermediate frequency is combined with constant frequencyoscillations from an oscillator 48. The output of the modulator 39includes two band-pass filters 4| and 44 in which one of the side-bandsof the modulation process is selected. Thus, the intermediate frequencymay be 455 kilocycles, the oscillator 40 may produce oscillations of 300kilocycles, and the filters 4| and 44 may be designed tov select theupper side-band of 755 kilocycles. Of course, any variations in thefrequency of the carrier wave received in the antenna 2| will bereflected in variations in the intermediate frequency carrier of 455kilocycles and consequently in variations in the Anominal755 kilocycleoutput of the modulator 39. As a result the filters 4| and 44 must havepass-bands wide enough to accommodate such frequency variations.

Included in the output path of the modulator 3S in addition to thefilters 4| and 44 is an amplifier 45. After selection and amplificationby the filters 4| and 44 and the amplifier 45, the 755 kilocycleside-band of the modulator 39 is sup- The 455 kilocycle intermediatefrequency output of the amplifier 28 is also supplied to this modulator42 through the circuit 36. One of the side-band products produced by themodulator 42 will be a frequency of 390 kilocycles. This product isselected by the filter 43 which is of the well-knowncrystal bridge typewhich may be designed to have a very narrow pass-band.

It will be noted in the codan circuit as so far described that thecircuit from the intermediate frequency amplifier to the modulator 42 isa directv connection l containing no network producinga phase shift, inthis sense being similar/to the circuit 6 ,of Fig. 1. Similarly thecircuit 3 1 between the intermediate frequency arnplifier 23 and themodulator 42 contains three networks, namely, the intermediate frequencyfilter 38 and the 755 kilocycle filters 4| and 44. Each of thesenetworks is a single section filter and produces a phase shift whichvaries progressively by about 1r radians across its pass-band. The threefil-ters in tandem insure a total phase shift `of somethingmore than21|- radians. The

filters correspond to elements 8 and Il in Fig. 1 and can be designed toproduce any desired phase shift or delay. The differential phasecharacteristic of the two paths to the modulator 42 can thus be socontrolled that the pseudo-carrier components in the output of themodulator 42 add to zero.

As has been pointed out above, these pseudocarrier components do notreplace the noise currents in the output of the modulator 42, but appearin addition thereto. The modulator output may also include productsproduced by the intermodulation of pairs of noise currents differing bynearly the'frequency of the carrier. While the lter 43 may be madehighly selective, there will usually be a certain Vnarrow band of noisecurrents which it will pass in addition to the 300 kilocycle carrier. Inorder to compensate for such noise currents, there is provided a secondpath from the modulator 42 which includes a band elimination filter 5lalso of the crystal bridge type. This filter will not pass the 300kilocycle carrier but only noise currents on each side thereof.

In order to make the codan operate when the carrier exceeds the noise bya fixed amount, the small amount of noise which comes through with the300 kilocycle carrier is separated from it by the band elimination lter5l. Such noise currents are rectified in the rectifier 46 and suppliedto a biasing winding 5D of the codan relay 48.v The 300 kilocyclecarrier output of the filter 43 is similarly rectified in the rectifier49 and supplied to the operating winding 41 of the codan relay 48.

When no carrier is being received, the relay 48 is held operated in onedirection (as shown) by the rectified noise output supplied to thewinding 50. In this position the radio receiver output is disabled bythe relay contacts 32 which short-circuit the audio output circuit 3l.When a carrier is received, the energy of the 300 kilocycle carrier inthe output of the lter 43 produces an opposing effect through thewinding 41 and causes the operation of the relay 48 in the oppositedirection. This opens the short-circuit on the line 3| at the relaycontacts 32, to complete the audio frequency output path of the receiverand render the circuit operative.

While the invention has been illustrated herein as specifically appliedto the codan circuit of a radio telephone receiver, it is clear thatitis not so limited but may be applied to other sysstems for example ICWradio telegraph receivers, test circuits, and the like.

What is claimed is:

1.l In a system for receiving signals subjected to noise, two signalpaths, one of said paths including a modulator for combining thereceived signals with oscillations of constant frequency, a secondmodulator for combining the outputs of said two paths, means in theoutput of said second modulator for selecting oscillations of saidconstant frequency, and means for providing such a difference intransmission delay through said two paths that the noise currents in theoutput of said first modulator do not combine with corresponding noisecurrents in the other path to produce a cumulative output from thesecond modulator of said constant frequency.

2.'. In a system for receiving radio signals subjected to noise, twosignal paths, one of said paths including a modulator for combining thereceived signals with oscillations of constant f requency to producemodulation products of frequency equal!to?thefsur'na'ndfdiirerfc ofiquencies of the s'ignal oscillations dt stant frequencyoscillatiorisfandL output of said modulato'r`fr-1 selectin one omodulation products, a secondi dulato combining the signal currents inthe other two paths with the modulation productsse Y by theafore-mentioned means, means in th'el'ou put of said second modulatorfor selecting osc lations of said constant frequency to the substantialexclusion of oscillations of otherfrequencies, and means for providingsuch a difference in transmission delay through said two paths that thecomponent modulation products produced by the combination ofcorresponding noise currents in the output of the first-mentionedselecting means and in said other path add with such phase relationshipas to produce substantially zero output from the second modulator ofsaid constant frequency.

3. In a system for receiving signals sub-ject to noise, two signalpaths, one of said paths including a modulator for combining thereceived signals with oscillations of constant frequency and filtermeans having a pass-band including the frequency of one of theside-bands of said modulation process and high attenuation forfrequencies of the modulating oscillations connected to the output ofsaid modulator, a second modulator for combining the output of said ltermeans and the signal oscillations in the other of said two paths, meansin the output of said second modulator for selecting oscillations of thefrequency of said constant frequency oscillations and substantiallyexcluding oscillations of other frequencies, and means in one of saidpaths for producing such a varying phase shift throughout the frequencyrange passed by said filter means that the total phase shift in saidpath differs from that in the other path by a factor of the order of 21rradians across said frequency range.

4. In a system for receiving signals subjectto noise and varying infrequency throughout a range of F cycles .per second,'two signal paths,one of said paths including a modulator for combining the receivedsignals with oscillations of constant frequency and lter means having apass-band of F cycles per second for selecting one of the side-bands ofthe modulation process to the exclusion of the modulating frequenciesand connected to the output of said modulator, a second modulator forcombining the output of said filter means with the output of the otherof modulator for selecting oscillations of the frequency of saidconstant frequency oscillations to the substantial exclusion ofoscillations of other frequencies, and means for providing a totaltransmission delay in one of -said paths differing from the totaltransmission delay of the other of said paths by the order of i seconds.

5.V In a system for receiving signals subjectto noise, two signal paths,one of said paths including a modulator for combining the receivedsignals with oscillations of a constant frequency and filter meanshaving a pass-band of F cycles per second for selecting one of theside-bands of said'modulation process to the exclusion of frequencies ofthe modulating oscillations and connected to the output of said.modulator, a second modulator for combining the' outputfof iis . saidtwo paths,` means in the output of said second said lter means with thesignal output of the other of said two paths, means in the output ofsaid second modulator for selecting oscillations of the frequency ofsaid constant frequency oscillations to the substantial exclusion ofoscillations of other frequencies, and means for providing a totaltransmission delay in one of said paths which differs from the totaltransmission delay of the other of said paths by substantially anintegral multiple of seconds.

6. In a system for receiving signals subject to noise and varying infrequency, two signal paths, one of said paths including a modulator forcombining the received signals with oscillations of constant frequencyand filter means connected to the output of said modulator and having apass-band substantially equal in width to the frequency variations ofthe received signal for selecting one side-band of the modulationprocess to the exclusion of the modulating oscillations, a secondmodulator for combining the output of said filter means with signaloscillations in the other of said two paths, and means in one of saidpaths for producing such a Varying phase shaft through the outputfrequency range passed by said filter means that the total phase shiftin said path differs from that in the other of said paths by a factor ofthe order of 21r radians across said frequency range.

'7. A radio receiver comprising means for combining the received radiosignals with local oscillations to produce intermediate frequencysignals, two paths for said intermediate frequency signals, a source ofconstant frequency oscillations, a modulator for combining theintermediate fre quency signals in one of said two paths with theconstant frequency oscillations from said source, filter means forpassing oscillations of a narrow band of frequencies including oneside-band of said modulation process and connected to the output of saidmodulator, a second modulator for combining the output of said ltermeans with the intermediate frequency signal output of the other of saidtwo paths, a second filter means l5 connected to the output of saidsecond modulator for selecting oscillations of the frequency of saidconstant frequency oscillations to the substantial exclusion ofoscillations of other frequencies, and means for producing invconjunction with said first filter means such a-progressive phase shiftover the band of frequencies passed by said first lter means that theVector sum of the modulation components of said constant frequencyproduced in said second modulator by the interaction of the noisecurrents in said band of frequencies and the corresponding noise:currents in the intermediate frequency signal band from said other ofsaid two paths is small in comparison with the algebraic sum of saidcomponents.

CARL B. H. FELDMAN.

